In recent years, the range of uses of compact ultraviolet light sources has been increasing; for example, new techniques to be applied to UV curing ink jet printers have been developed.
Known as a compact ultraviolet light source is an ultraviolet light emitting diode (LED) which employs, for example, a gallium nitride (GaN) based compound semiconductor. In such an ultraviolet LED, a composition ratio of aluminum (Al) in a GaN based compound semiconductor containing Al which forms an active layer may be changed, so that an emission wavelength is adjusted in the ultraviolet region of 380 nm or shorter, for example.
Under the circumstances, however, the ultraviolet LED becomes low in external quantum efficiency according to non-radiative transition due to defect in a semiconductor crystal, and to carrier overflow in the active layer and resistance loss from the construction that requires a p-type layer, which cannot but become a low carrier density by a high activation energy of a p-type impurity such as Mg, and is thus not practical.
Furthermore, known as an ultraviolet light source employing a semiconductor device is one which allows a semiconductor multi-layered film element to emit light by irradiating the semiconductor multi-layered film element with an electron beam from an electron beam irradiation source (see Patent Literature 1).
Since such an ultraviolet light source does not require the formation of a p-type semiconductor layer of an element inevitable for the LED, the ultraviolet light source is not affected by the quality of a p-type semiconductor layer, whereby it is possible to provide an ultraviolet light source which is capable of emitting ultraviolet radiation with stability.
However, the aforementioned ultraviolet light source has the following problem. To allow the semiconductor multi-layered film element to emit light with high efficiency, the semiconductor multi-layered film element needs to be irradiated with an electron beam accelerated by an acceleration voltage, for example, of a few tens of kV or greater, thereby likely causing the semiconductor multi-layered film element to emit an X-ray. Thus, if the element is used as the ultraviolet light source, the light source requires a structure for shielding the X-ray, which makes it difficult to provide a compact ultraviolet light source.